Truck Mystery: Stalling After Power Cycles

The Mighty Four Banger

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I’ve helped a friend deal with his old Chevy on multiple occasions. While we usually have it figured out, there’s always been something that’s stumped me. After reconnecting a battery, it will have problems not being able to idle and will just stall. This problem can be remedied by going for a moderate cruise on the highway and after some time it all goes back to normal. This can be a massive headache as jumpstarting a dead battery requires someone to be mashing on the gas right after it fires up and you can’t leave it alone or else it will promptly die setting you back to square one.
Why would it be like this?

my guess is just that the ECU loses its memory and needs to relearn wtf is going on so it can keep the engine running. I just wouldn’t expect it to initially fail so miserably.

I know my operators manual says things like the transmission forgets where to shift the best so changing batteries can cause rough shifting for a while and how I lose all my radio and climate settings each time, but I’ve never had or heard of any engine issues like this before.

could old parts be a problem? My one idea is the ECU is just resetting to default conditions and has to re learn how to support the worn out parts. Maybe this could be mostly fixed with new spark plugs or fuel injectors? Since it’s not mine I’d want to be pretty certain of a fix before I recommend something.
 

aim4squirrels

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These are total stabs in the dark over the internet, so take it for what it's worth...

How old is the vehicle?

I would not blame the ECU initially. Default programming on an ECU is always to run a bit rich then take readings from the o2 sensors to lean out fuel trims to be more economical.

Does it have an IAC valve on the throttle body? Have you cleaned the throttle body and the IAC port recently? Check the IAC for proper functioning as well.

It could be a bad coil pack.

It could also be a failing crank position sensor.

Tough to diagnose car problems over the internet, and I'd hate to lead you down the path of throwing parts at it.
 

poolmon

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Sounds like attaching an inexpensive diagnostics tool would be a first step trying to narrow the issue.
Forgetting learned memory should result in default settings and starting should be a breeze. On the other hand, a faulty module (like throttle position sensor) could be misreporting to the ECU.
 
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OBeerWANKenobi

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What year and model is this mystery Chevy? That could help a lot with some diagnosis. I'm pretty familiar with the 99 through 2007.5.

If it eventually gets back to running fairly smoothly I wouldn't be looking at the ECU as the problem, first anyway.

Check and make sure the MAF sensor is okay. If the ECU loses its fuel trims and has to relearn that upon startup it will first look to the MAF sensor to see if he can use that to determine the air fuel ratio. If there's something wrong with it and that's no dice it will revert to a speed density style and look to the MAP sensor and IAT sensor. It can run like that but it might take a while for it to get working right. It also should throw a code.

The crank sensor is definitely another possibility but usually you'll see that acting up on other restarts and not just after messing with the battery. I had to replace the crank sensor and do a crank sensor relearn on mine last year.

MAF, MAP, IAT, TPS. Are the major checks and fairly easy to check. IAT is probably actually in your MAF.

Anyway, a little more information might be helpful. If it does actually turn out being the ECU and it's in the year range that I mentioned above you can easily get one for like 50 bucks out of a junkyard.
 

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poolmon

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Because you indicate it is an old truck and happens after reapplying electrical power, it could be something as innocuous as a faulty or intermittent ground on a module/sensor that after running and reheating reconnects sufficiently.

When checking for proper electrical connectivity an incandescent test lamp should used and not a digital multi-meter or led indicator to be sure that sufficient amperage is present, and not just voltage. Below is a video covering that for anyone who may not have seen it before (ignore title of the video as it contains more than what is indicated) :

Edit: The sensors that control Fuel Trim (as mentioned above) would be first to be suspect.

 
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The Mighty Four Banger

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I wasn’t thinking it was a bad ECU, just that it puzzles me how it couldn’t keep the vehicle running. I see where one could’ve made that assumption from me though I think it’s an 04 1500 with one of the V8s.

in my independent research I have began to also suspect a dirty throttle body. I didn’t think about it possibly being a bad sensor.

the issue usually takes half a day or so to work itself out and I guess he’s good at not killing the battery. This only came up in my head again because he took the battery off when we replaced the ignition switch a couple days ago.
 

OBeerWANKenobi

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I wasn’t thinking it was a bad ECU, just that it puzzles me how it couldn’t keep the vehicle running. I see where one could’ve made that assumption from me though I think it’s an 04 1500 with one of the V8s.

in my independent research I have began to also suspect a dirty throttle body. I didn’t think about it possibly being a bad sensor.

the issue usually takes half a day or so to work itself out and I guess he’s good at not killing the battery. This only came up in my head again because he took the battery off when we replaced the ignition switch a couple days ago.
Cool, if it's an '04, everything I mentioned is relevant. First, is the check engine light on? If it is, read the codes or get them read for free at the local autoparts store. '04 is the first year of DBW (Drive By Wire) throttle in those trucks so if it does have that it won't have an IAC (Idle Air Control) valve. The DBW throttle takes care of that.

Also, as I mentioned before, don't sweat it if it does happen to be an ECU problem. You can get one cheap on car-part.com . I've even got extras for these trucks just laying around. LOL. It should be a "Blue Green" P59.
 

The Mighty Four Banger

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My diagnostic tool was taken so I still want to track that down, Then I guess I’ll start testing away.

this only happens whenever the battery gets disconnected and it’s already been resolved since the last time it was so there’s not really much incentive to attack it right now.

thanks all, I’ll be back if it happens again and I get any updates.
 

poolmon

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When it happens again it sounds like a loss of long term fuel trim (LTFT) data. Power cycling the PCM causes loss of the LTFT data (see below). Problem is LTFT gets its stored data over time from the short term fuel trim (STFT) readings. So, it would appear that STFT is currently incorrect and the vehicle is correcting for an existing problem but forgetting the correction when power cycled. Good luck.

LT Fuel Trim.JPG
 

aim4squirrels

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Of there are no DTCs to work with, start by cleaning the throttle body (be sure to remove it and clean the back of the plate as well) and also clean the MAF sensor. I believe CRC makes a product that is literally called MAF sensor cleaner. This will be the cheapest 2 first tries. Also, I would scour some Chevy truck forums to look for MAF sensor cleaning threads. Some MAF sensors don't respond well to cleaning and just need to be replaced. If that's the case, owners of a truck that old will have figured it out a while ago and will have posted as such to truck owners inquiring about it.

If it's only happening after a PCM dump of LTFT data then it sounds to me like the preprogrammed throttle position initial tip in setting is inadequate to keep the vehicle running and as the PCM/ECM learns this, it is opening the plate more to compensate and maintain the preprogrammed RPM at idle. The question is, is it because of a dirty TB with filth built up at the tip in point, or bad data from your MAF sensor causing the PCM to incorrectly guess how much to open the throttle to keep the truck running. Eventually, the PCM can ignore MAF data and default to a less precise MAP process as @OBeerWANKenobi mentioned to keep the truck running.

I'd start there for the cheap fix that couldn't hurt to do anyway.

It could be the throttle position sensor, but I would think it should throw a code for erroneous data being reported in the DBW system. And yes, you can throw out the IAC info because drive by wire systems just control the throttle plate electronically to maintain idle. In essence, the throttle plate is the IAC, because it can be computer controlled. Be careful when you clean the throttle body not to manually jack with the throttle plates, because some vehicle's throttle position sensors don't like it when the plates are moved manually.


You can likely use the MAF sensor cleaner on the TB, or if it's really grimy seafoam makes a good product called deep creep that's basically a foaming version of seafoam that can bust up carbon and dirt and it won't hurt if any of it gets sucked into the intake.

As a last resort you could even try brake parts cleaner if you're judicious about where you spray it and keep it away from sensors and let it dry completely before you fire up the truck. It doesn't take long to evaporate.
 

Jaguar

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Power foam baby, just stuff this down the intake while running the engine and watch a plume of carbon exit :)
 

gwing

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From my AHM days the learned values are stored in EEPROM, but don't typically get erased by just unplugging the battery. If you hook up the diag tool, you should be able to see the learned values. Disconnect the battery and reconnect it and see if the learned values change. Learned values include STFT & LTFT, but these are mainly used to protect the cats from a lean situation. For rough idle, look for learned crank pulse values and throttle settings.
 

rider9195

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Might want to look at and clean the throttle body out and perform an idle learn. Higher end scanners can usually perform the learn, but I know there is a manual procedure for it too.

This happens when replacing the ECM on certain year GM vehicles. Throttle body is dirty, new ECM doesn't know and has a base learn so when it's started it will run rough and stall out at idle.

Sent from my SM-G950U using Tapatalk
 

Choose Life

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https://www.topjdmstore.com/products/mega-raizin-voltage-stabilizer-12v I know this looks like a bit of a bs item but it works awesome on my truck for stabilizing crank up voltage. It's like a permanent one of those jumper packs attached to your battery. I get zero flicker on my dash, might help with weak electric system problems
Have you tried installing an alternator
So many times I've had weak power, flickering lights and ever so slightly low voltage (half to one volt) and its always a failing alternator
 

Jaguar

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Have you tried installing an alternator
So many times I've had weak power, flickering lights and ever so slightly low voltage (half to one volt) and its always a failing alternator
Alternators fine, it boosts cranking amps, which now run all the extra crap I have, an intercooler pump, etc.
 

SwankPeRFection

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@aim4squirrels is most likely correct on this if the engine has a drive by wire TB and it’s dirty or has codes for a bad O2 sensor or best put a bad O2 sensor heater circuit on the primary. The short terms fuel trims have to be relearned when an ECU loses power, but the base maps shouldn’t idle like crap, so it’s a dirty TB, MAF (if equipped), etc. DTCs reset natively when an ECU loses power, so if you’ve got a CEL on startup, then you’ve got some issue somewhere that you need to fix that most likely causing your rough initial idle.
 

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